This invention relates generally to rotation sensors and particularly to fiber optic rotation sensors. Still more particularly, this invention relates to apparatus and methods for processing the output signal from a fiber optic rotation sensor to null the rotation-induced phase shift between counterpropagating light waves in a Sagnac ring and to determine the rotation rate of the sensor.
A fiber optic ring interferometer typically comprises a loop of fiber optic material having counter-propagating light waves therein. According to the Sagnac effect the wave traveling in the direction of rotation of the loop has a longer transit time through the loop than the wave traveling opposite to the direction of rotation. This difference in transit time is seen as a shift in the relative phases of the waves. The amount of phase shift depends on the rotation rate. After traversing the loop, the counter-propagating waves are combined so that the interfere to form an optical output signal. The intensity of the optical output signal varies as a function of the type and amount of interference, which is dependent upon the relative phase of the counter-propagating waves. The optical output signal produced by the interference of the counter-propagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing it to determine the rotation rate as a function of the phase shift.
In order to be suitable for inertial navigation applications, a rotation sensor must have a very wide dynamic range. The rotation sensor must be capable of detecting rotation rates as low as 0.01 degrees per hour and as high as 1,000 degrees per second. The ratio of the upper limit lower limits to be measured is approximately 10.sup.9.
It has been found that the dynamic range of a fiber optic rotation sensor to may be increased by applying a feedback signal and to the waves in the sensing coil to null the Sagnac phase shift. A signal indicative of the amount of feedback signal required to null the Sagnac shift may be processed to determine the rotation rate.
Previous signal processing techniques are unduly complex, expensive and inaccurate over the dynamic range required for a navigation grade rotation sensor. Previous signal processing techniques provide inaccurate results at low rotation rates because phase modulators typically are nonlinear devices that cause scale factor nonlinearity.